How to find entities that do not have a particular attribute?

gistfile1.clj

(use '[datomic.api :only [q db] :as d]
     'clojure.pprint
     'clojure.repl)

(do
  (def *uri "datomic:mem://likes")
  (d/create-database *uri)
  (def *conn (d/connect *uri)))

;; define utilities
(do
  (defmacro tx [ll#]
    `(d/transact *conn ~ll#))

 (defmacro ent [eid]
   `(d/entity (db *conn) ~eid))

 (defmacro f [query]
   `(q ~query (db *conn)))

 (defmacro p [form]
   `(-> ~form seq pprint)))



;; define schema
(tx [{:db/id #db/id[:db.part/db]
      :db/ident :pers/age
      :db/valueType :db.type/long
      :db/cardinality :db.cardinality/one
      :db/doc "Age of the person."
      :db.install/_attribute :db.part/db}
              
     {:db/id #db/id[:db.part/db]
      :db/ident :pers/likes
      :db/valueType :db.type/keyword
      :db/cardinality :db.cardinality/many
      :db/doc "Who likes who/ what."
      :db.install/_attribute :db.part/db}

     {:db/id #db/id[:db.part/db]
      :db/ident :pers/name
      :db/valueType :db.type/keyword
      :db/cardinality :db.cardinality/one
      :db/doc "Name as keyword"
      :db.install/_attribute :db.part/db}
                
     {:db/id #db/id[:db.part/db]
      :db/ident :likes
      :db.install/_partition :db.part/db}])

;; data
(tx [{:db/id #db/id [:db.part/user]
      :pers/name :sally
      :pers/age 21
      :pers/likes :opera}

     {:db/id #db/id [:db.part/user]
      :pers/name :fred
      :pers/age 42
      :pers/likes :pizza}

     {:db/id #db/id [:db.part/user]
      :pers/name :ethel
      :pers/age 42
      :pers/likes :sushi}

     {:db/id #db/id [:db.part/user]
      :pers/name :john
      :pers/age 40}])


(do
 (tx
   [{:db/id #db/id [:db.part/user]
     :db/ident :attr-missing?
     :db/fn (d/function
             {:lang :clojure
              :params '[db pers# attrib#]
              :code '(-> db (datomic.api/entity pers#) attrib# nil?)})}])
 
 (def attr-missing? (-> *conn db (d/entity :attr-missing?) :db/fn))
)


(do
  (tx
    [{:db/id #db/id [:db.part/user]
      :db/ident :attr-exists?
      :db/fn (d/function
              {:lang :clojure
               :params '[db pers# attrib#]
               :code '(-> db (datomic.api/entity pers#) attrib# nil? not)})}])

  (def attr-exists? (-> *conn db (d/entity :attr-exists?) :db/fn))
)

(p (f '[:find ?name :where
      [ ?e :pers/name ?name]]))
;; ([:john] [:sally] [:ethel] [:fred])


(p (f '[:find ?name :where
      [ ?e :pers/name ?name]
      [(user/attr-exists? $ ?e :pers/likes)]]))
;; ([:sally] [:ethel] [:fred])

(p (f '[:find ?name :where
      [ ?e :pers/name ?name]
      [(user/attr-missing? $ ?e :pers/likes)]]))
;; ([:john])